Addition of silver nanoparticles to the zinc ferrite/polyaniline composition for boosting its visible photocatalytic degradation

Enhancing the photocatalytic activity of ZnFe2O4 with a good energy band gap to degrade industrial waste under sunlight illumination can help to develop green environments. Here, to improve the photocatalytic efficiency of ZnFe2O4 ferrites, they were merged with polyaniline (PAni) and silver (Ag) nanoparticles to synthesize Ag@ZnFe2O4–PAni plasmonic nanostructures. The as-synthesized nanostructures were characterized using a series of advanced characterization techniques to confirm successful formation and investigate photocatalytic improvement origins. It was found that incorporating Ag NPs along with the PAni to ZnFe2O4 increases its absorption power and red-shifts its energy band gap, which increases the electron–hole production rate by exposure to light in ZnFe2O4. Contribution of the surface plasmon resonance effect of Ag NPs and conjugated double bonds of PAni to charge transfer mechanisms in Ag@ZnFe2O4–PAni material increased charge separation during photocatalytic process, boosting the photodegradation performance of ZnFe2O4.


Introduction
With the increase in world population and the industrial development to meet human needs for food, clothing, electrical appliances, etc., the increase in environmental pollution has become a critical challenge. 1,2This industrial waste affects the water quality of rivers and underground water, and subsequently negatively impact human health and the ecosystem. 3,46][7][8] For years, the Environmental Protection Agency (EPA) and World Health Organization (WHO) have recommended that the world's industries do not discharge contaminated water into their surrounding environments.0][11][12][13] These approaches have serious drawbacks, including toxic products, high operating costs, and low elimination rates.In contrast, advanced oxidation photocatalysis offers an ultimate deterioration, cheap, and eco-friendly method to treat wastewater with considerable recyclability. 14,15he photocatalysis process is used to disinfect the wastewater and eliminate contaminants from water and air.The photocatalyst material promotes the elimination reaction rate of targeted waste from water with the assistance of a light illumination source.During the process, the irradiated light reacts with the photocatalyst material to generate electron-hole pairs.][18] In recent years, nano-sized materials were introduced as a primary area of research to nd advanced technologies for meeting the world's demands.Nano-sized materials have a considerable surface area, which increases their chemical reaction with other materials.In addition, tunable electrical conductivity and optical properties of nano-sized materials along with their mechanical strength, compared to their bulk counterparts, makes them interesting materials to design different advanced technologies.0][21][22][23] As mentioned, industrial development and its conict of interest with the green environment have led to an excessive increase in the pollution of natural water.Developing advanced nanomaterial photocatalysts has emerged as a promising technology to address this concern. 246][27][28][29] Zinc ferrite (ZnFe 2 O 4 ), due to their small band gap, is more favorable for visible photocatalytic degradation of water pollution.][32] Yu et al. 31 to enhance catalytic activity of zinc ferrites substituted copper metal to ZnFe 2 O 4 structure.They deduced that the increased catalytic activity of zinc-based ferrite ascribes to the dual active sites of Fe and Cu and oxygen vacancies aer Cu substitution.Janani et al. 32 decorated ZnFe 2 O 4 with CdO material to boost the photocatalytic efficiency of ZnFe 2 O 4 .They found that the CdO/ZnFe 2 O 4 with a high surface area offers more active sites to induce the photocatalysis performance of the system.In addition, they concluded that coupling of ZnFe 2 O 4 and CdO improved the lifetime of charge carriers and promoted the reaction of photo-generated electrons and holes with dye molecules.Akshhayya et al. 33 decorated ZnFe 2 O 4 with SnS 2 material to accelerate visible light photocatalysis of methylene blue.They observed that the formed interfacial contact in the ZnFe 2 O 4 /SnS 2 system suppresses charge recombination and retains better charge separation.These improvements with high visible-light absorbing ability offer an n-SnS 2 / p-ZnFe 2 O 4 hybrid system with improved photocatalytic activity.Kaushal et al. 34 developed a ZnFe 2 O 4 @nitrogen-doped carbon dots hybrid material to decomposite ciprooxacin and nor-oxacin materials under visible light irradiance.They observed that nitrogen-doped carbon dots hybridized with zinc ferrites, which enhanced photocurrent density and surface area.The ZnFe 2 O 4 @nitrogen-doped carbon dots exhibited efficient transfer of charge carriers and can be used for environmental remediation through the photocatalytic degradation process.][37] Photosensitizer behavior of PAni under visible light illumination establishes the electron donor phenomenon in it, resulting in enhanced catalytic performance in PAni-contained hybrid heterostructures. 38,39Developing plasmonic nanostructures based on ZnFe 2 O 4 also can be used to increase the ZnFe 2 O 4 catalyst activity.][42] The objective of the current study is to develop a ternary plasmonic nanostructures system based on ZnFe 2 O 4 through its combination with PAni and Ag NPs.The Ag@ZnFe 2 O 4 -PAni hybrid system was employed for the visible light photocatalytic application and recorded promising results.Results showed that the Ag@ZnFe 2 O 4 -PAni nanocomposites have efficient charge transfer, higher surface area, and higher light-harvesting ability due to synergistic effects of Ag NPs and PAni, resulting in higher photodegradation behavior than the ZnFe 2 O 4 and ZnFe 2 O 4 -PAni materials.Moreover, the effect of the pH value of aqueous mediums, the initial concentration of dye, and the amount of photocatalyst were examined on the photocatalytic performance of the Ag@ZnFe 2 O 4 -PAni.

Materials
All solvents and materials that used in this study were purchased from Merck and used as received without any more purication steps.

Synthesis of zinc ferrite material
To synthesize ZnFe 2 O 4 , 1.8 mmol of Zn(CH 3 COO) 2 $2H 2 O and 2 mmol of Fe(C 5 H 7 O 2 ) 3 were dissolved in 10 mL of ethanol.In another vial, 10 mL of 2 mmol tetramethylammonium hydroxide pentahydrate (TMAH) solution in ethanol was prepared.Then, two solutions were added together and sonicated to obtain a dark red color solution.The obtained solution was exposed to microwave radiation at 180 °C for 45 min.The product was centrifuged and washed with ethanol.Finally, the brown precipitates were dried in a vacuum oven at 60 °C overnight, followed by calcination at 400 °C for 2 h to produce ZnFe 2 O 4 NPs.

Synthesis of zinc ferrite-polyaniline material
To synthesize ZnFe 2 O 4 -PAni nanocomposite, an in situ polymerization technique was employed.4 g of aniline material was dissolved into 200 mL of 1 M HCl solution by stirring for 2 h at a temperature of 50 °C.Then, 1.2 g ZnFe 2 O 4 NPs were added to the aniline solution, followed by sonication for about 30 min at room temperature to homogenize the scattered ZnFe 2 O 4 NPs.In another vial, 100 mL of 1 M HCl was carefully mixed with ammonium persulfate (APS) to obtain a 0.2 M solution.Then, the precooled APS solution was dropped wise to the aniline-ZnFe 2 O 4 solution, followed by stirring at a temperature of 2 °C for 12 h to complete polymerization.The obtained solution was kept in the freezer overnight.Finally, the solution was ltered and washed with deionized water and methanol, followed by drying in a vacuum oven for 24 h at a temperature of 60 °C.

Synthesis of zinc ferrite-polyaniline decorated with silver nanoparticles
The Ag@ZnFe 2 O 4 -PAni plasmonic nanocomposites were synthesized through Ag + ions photoreduction in the ZnFe 2 O 4 -PAni aqueous solution (see Fig. 1).400 mg of ZnFe 2 O 4 -PAni nanostructures were sonicated in 100 mL deionized water for 30 min to prepare a homogeneous suspension.Then, 40 mL of 2.5, 5, and 10 mM of AgNO 3 solution in deionized water were quickly added to ZnFe 2 O 4 -PAni aqueous solution in dark conditions.The suspensions were stirred for 2 h at room temperature in dark conditions and then exposed to natural sunlight for 90 min.The black precipitates were centrifuged and washed with deionized water and methanol repeatedly to remove impurities.The washed products were dried overnight in an oven at 60 °C for 24 h.The obtained powders were marked as AZP1, AZP2, and AZP3, referring to samples prepared using 2.5, 5, and 10 mM AgNO 3 solutions, respectively.

Photocatalytic degradation investigation
The photocatalytic performance of the synthesized nanostructures for methylene blue (MB) or rhodamine B (RhB) dyes photodegradation was investigated by recording their UV-Vis spectra over reaction time under sunlight irradiance.For this test, 25 mg of MB or RhB dyes were dissolved into distilled water to obtain 25 ppm solutions.Then, 10 mg of photocatalyst materials (ZnFe 2 O 4 , ZnFe 2 O 4 -PAni, AZP1, AZP2, and AZP3) were dispersed into 100 mL dye solutions.The dye/photocatalyst material solutions were stirred under dark conditions for 30 min to establish an adsorption/desorption equilibrium between dye molecules and photocatalyst materials.Aerward, solutions were exposed to simulated sunlight illumination, and recorded their absorbance spectra.The same procedures were conducted to study the effects of pH and catalyst material concentration on the photodegradation performance of AZP2 material.

Characterizations
Crystalline structure of ZnFe 2 O 4 , ZnFe 2 O 4 -PAni, AZP1, AZP2, and AZP3 materials were measured by recording X-ray Diffraction (XRD) using a Rigaku Ultima IV XRD instrument.The morphology of the synthesized nanostructures were monitored using CM120 TEM and TESCAN Mira III FESEM.The FTIR spectra of materials were investigated using NICOLET-IS FTIR instrument.UV-vis spectra of samples were recorded using PerkinElmer Lambda Spectrometer.RAMAN spectra of materials were collected using a WiTec alpha 300 Raman spectrometer.The EIS response of samples were measured using Gamry Interface 1000 Potentiostat.PL spectra of material was collected using FLS 1000, Edinburgh spectrometer.Specic surface area of samples were measured by recording their Bet response using a BELSORP Mini II device.The transient photocurrent response of samples were measured using a PGSTAT302 N electrochemical workstation with a 500 W xenon lamp as the light source.The FESEM images of samples were taken to investigate the surface morphology of ZnFe 2 O 4 and AZP2 photocatalysts (Fig. 4a and b).The nano-sized ZnFe 2 O 4 particles were aggregated due to the magnetic attraction forces among ZnFe 2 O 4 (Fig. 4a).In Fig. 4b, it is evident that particles are highly agglomerated, which is possibly due to the incorporation of Ag NPs.The successful incorporation of Ag NPs into the ZnFe 2 O 4 -PAni nanostructures was evaluated by TEM (Fig. 3c).As can be seen, ZnFe Nyquist plots of samples using EIS method were obtained under light illumination to investigate the interfacial charge transfer resistance (Fig. 6a).By tting the EIS spectra with the equivalent electrical circuit, parameters including series resistance (R S ) and charge transport resistance (R Ch ) were obtained.The EIS arc radius of AZP2 on the Nyquist plot is smaller than that of ZnFe    Fig. 7a shows PL spectra of ZnFe 2 O 4 and AZP2 samples.The emission peak observed for both samples observed at 451 nm and indicates to the charge recombination. 45As seen, the AZP2 has a weaken PL intensity than the pure ZnFe 2 O 4 , suggesting  The solution pH value plays an important role in the photocatalytic-based reactions.The pH adjusting alters the interaction between the photocatalyst and dye molecules.Here, the photocatalytic activity of AZP2 nanostructures enhances from 78.1% to 99.6% as the pH value increases from 4 to 7 (Fig. 9a).By increasing the pH value to non-acidic conditions, the photocatalytic efficiency reduces to 89.2 (pH 11).Under acidic conditions (pH < 7), both Ag@ZnFe 2 O 4 -PAni and MB are positively charged, which results in a repulsive interaction in the solution and reduces photocatalytic activity.In neutral conditions, appropriate interactions between positively charged photocatalysts and MB enhance the photocatalytic activity.Under alkaline conditions, a slight reduction in the photocatalytic efficiency is observed, which is due to the interactions between anionic MB negatively charged photocatalysts.Next, effect of the initial AZP2 amount on the photocatalytic activity was investigated (Fig. 9b).For this aim, 5, 10, 15, 25, 40, and 50 mg of AZP2 nanostructures were dispersed in 100 mL of 25 ppm MB aqueous solution and their photocatalytic activities over 60 min light illumination were monitored.By enhancing the AZP2 amount, the MB photodegradation efficiency increased.The MB photodegradation efficiency started declining by raising the AZP2 amount to >40 mg, possibly due to the photocatalysts aggregation or more light scattering from photocatalyst nanoparticles.

Results and discussion
Furthermore, effect of the initial MB concentration on the AZP2 photocatalytic performance was studied by varying MB concentration from 10 to 50 mg L −1 and probing their degradation efficiency.As shown in Fig. 9c, the photodegradation performance reduced from 100% (for 10 mg L −1 ) to 88.1% (50 mg L −1 ).The observed decline in performance is possibly due to the saturation of photocatalytic sites of AZP2 in high amounts of MB.
Reusability experiments were conducted for six successive runs (Fig. 10).Aer each photodegradation run, the AZP2contained solution was centrifuged at 4000 rpm for 6 min.The collected AZP2 was washed with deionized water and methanol, followed by drying for 6 h at 75 °C.The dried AZP2 was again used for reusability test.As can be seen, a photocatalytic performance of 99.6% and 96.6% was recorded for the rst and second cycles; however, in total a ∼17% decline in photocatalytic performance was observed aer six runs.The reusability test implies that the Ag@ZnFe 2 O 4 -PAni plasmonic nanocomposites can be used for photodegradation of industrial aqueous wastes.
The possible photodegradation mechanism of the Ag@ZnFe 2 O 4 -PAni system is depicted in Fig. 11.By exposing sunlight illumination to the Ag@ZnFe 2 O 4 -PAni/dye aqueous solution, the ZnFe 2 O 4 valence band (VB) electrons absorb light energy and generate hot electrons.Next, these hot electrons inject into the conduction band of PAni polymer, then moved to Ag NPs surface.Theses hot electrons react with surrounding O 2 and form radicals (O 2 c).The O 2 c radicals assist to form the hydroxyl radicals (cOH).In addition, the holes from the PAni molecules transfer to the VB of ZnFe 2 O 4 , which contribute to the cOH radicals formation.Then, the cOH and O 2 c radicals decompose the dye molecules.
Table 1 summarizes the kinetic rate values provided for several types of nanocomposites and compares them with the values acquired in this work.

Conclusions
In the current study, ZnFe 2 O 4 photocatalytic activity was increased by developing Ag@ZnFe 2 O 4 -PAni plasmonic nanostructures.The synthesized Ag@ZnFe 2 O 4 -PAni nanostructures was employed to photodegradation of MB and RhB dyes under simulated sunlight illumination.Results showed the ternary nanostructures exhibit higher photocatalytic efficiency than that of the pure ZnFe 2 O 4 ferrites.Aer 60 min light illumination, Ag@ZnFe 2 O 4 -PAni plasmonic nanostructures decomposed 99.6% of MB dye.Incorporating Ag NPs into to ZnFe 2 O 4 -PAni nanocomposites boosted light-harvesting photocatalyst and reduced energy bandgap from 2.01 eV to 1.92 eV.These phenomenons increased the electron-hole production rate in ZnFe 2 O 4 by exposure it to light.Moreover, contribution Ag NPs and PAni to charge transfer mechanisms boosted charge separation during photocatalytic process.The Ag@ZnFe 2 O 4 -PAni plasmonic nanostructures offered larger surface area and photocatalytic sites than the pure ZnFe 2 O 4 .Overall, enlarged surface area, increased electron-hole production rate, and boosted charge separation are the origins of photocatalytic improvement of ZnFe 2 O 4 .Furthermore, the Ag@ZnFe 2 O 4 -PAni photocatalyst with its considerable reusability can be used in industries to photodegrade their aqueous wastes.In addition, the obtained results show that by modication ZnFe 2 O 4 materials with metallic dopants and by developing ZnFe 2 O 4 -polymer hybrid systems can design efficient photocatalyst materials to waste water treatments.

Fig. 2
Fig.2shows XRD patterns of different samples to conrm their successful synthesis.As shown in Fig.2a, there are six peaks

2 O 4
NPs were capped with PAni molecules.Small Ag NPs attached on the ZnFe 2 O 4 or PAni surfaces.Fig. 4d shows the EDS spectrum of the Ag@ZnFe 2 O 4 -PAni nanostructure.As seen, six elements of C, N, O, Fe, Zn, and Ag were observed in the sample, consistent with existing elements in the Ag@ZnFe 2 O 4 -PAni composite.It proves successful synthesis of the Ag@ZnFe 2 O 4 -PAni.Fig. 5a shows UV-Vis spectra of as-prepared photocatalyst materials.As can be seen, by incorporating Ag NPs into ZnFe 2 O 4 -PAni nanostructure, its light-harvesting behavior is increased.This phenomenon increases the potential of ZnFe 2 O 4 to absorb exposed light and generate electronhole charges.Fig. 5b depicts the corresponding Tauc plot of samples to investigate their optical bandgap energy.The optical bandgap for ZnFe 2 O 4 is 2.01 eV, aligning with the reported value in literature. 44Through the reaction of ZnFe 2 O 4 with PAni materials, the optical bandgap is red-shied to 1.97 eV and by the incorporation of Ag NPs to ZnFe 2 O 4 -PAni, it more reduced to 1.92 eV.It reveals that forming Ag@ZnFe 2 O 4 -PAni nanostructures can minimize the optical band gap of net ZnFe 2 O 4 and increase light utilization, improving the photodegradation performance of ZnFe 2 O 4 under visible light illumination.
2 O 4 and ZnFe 2 O 4 -PAni, which implies the AZP2 nanostructures have the lowest R Ch value (87.8 U).It indicates a fast interfacial charge carrier transfer with high photogenerated carriers separation efficiency, which both are responsible for the enhanced photocatalytic activity of ZnFe 2 O 4 .It is due to the contribution of conjugated double bonds along the PAni structure and SPR effects of Ag NPs for the charge transfer mechanisms in AZP2 material.To get deeper insight on the photoelectrochemical properties of ZnFe 2 O 4 NPs before and
aer modications with PAni and Ag NPs, transient photocurrent response of ZnFe 2 O 4 and ZnFe 2 O 4 -PAni, and AZP2 materials were investigated (Fig. 6b).As observed, the ZnFe 2 O 4 sample implies a delayed photocurrent response upon the light is turned on and off.In contrast, the ZnFe 2 O 4 -PAni and AZP2 materials have rapid photocurrent responses.Moreover, results shows that the photocurrent of the ZnFe 2 O 4 -PAni electrode (0.213 mA cm −2 ) is 67.7% higher than that of ZnFe 2 O 4 (0.126 mA cm −2 ), indicating the improved photoinduced electron-hole pairs separation efficiency in ZnFe 2 O 4 -PAni due to unique interfacial charge transfer of heterojunction.AZP2 shows the highest photocurrent density (0.358 mA cm −2 ), indicating that the charge carrier separation in modied nanostructure of Ag@ZnFe 2 O 4 -PAni is further improved.

Fig. 9 Fig. 10
Fig. 9 (a) Effect of pH solution, (b) effect of photocatalyst amount and (c) effect of initial MB concentration on the photodegradation performance for AZP2.